Implantable stimulation devices generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. patent application Ser. No. 11/177,503, which is now U.S. Pat. No. 8,606,362 issued on Dec. 10, 2013.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. An SCS system typically includes an Implantable Pulse Generator (IPG), electrodes, at least one electrode lead, and, optionally, at least one electrode lead extension. As shown in FIG. 1, the electrodes 106, which reside on a distal end of the electrode lead 102, are typically implanted along the dura 70 of the spinal cord 19, and the IPG 100 generates electrical pulses that are delivered through the electrodes 106 to the nerve fibers within the spinal column 19. Electrodes 106 are arranged in a desired pattern and spacing to create an electrode array 110. Individual wires 112 within one or more electrode leads 102 connect with each electrode 106 in the array 110. The electrode lead(s) 102 exit the spinal column 19 and may attach to one or more electrode lead extensions 120. The electrode lead extensions 120, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG 100 is implanted. Alternatively, the electrode lead 102 may directly connect with the IPG 100.
As should be obvious, an IPG needs electrical power to function. Such power can be provided in several different ways, such as through the use of a rechargeable or non-rechargeable battery or through electromagnetic (EM) induction provided from an external charger, or from combinations of these and other approaches, which are discussed in further detail in U.S. Pat. No. 6,553,263 (“the '263 patent”). Perhaps the favorite of these approaches is to use a rechargeable battery in the IPG, such as a lithium-ion battery or a lithium-ion polymer battery. Such a rechargeable battery can generally supply sufficient power to run an IPG for a sufficient period (e.g., a day or more) between recharging. Recharging can occur through the use of EM induction, in which EM fields are sent by an external charger to the IPG. Thus, when the battery needs recharging, the patient in which the IPG is implanted can activate the external charger to transcutaneously (i.e., through the patient's flesh) charge the battery (e.g., at night when the patient is sleeping or during other convenient periods).
The basics of such a system are shown in FIG. 2. As shown, the system comprises, in relevant part, the external charger 208 and IPG 100. A primary coil 130 in the charger 208 produces an EM field 290 capable of transcutaneous transmission through a patient's flesh 278. The external charger 208 may be powered by any known means, such as via a battery or by plugging into a wall outlet, for example. The EM field 290 is met at the IPG 100 by another coil 270, and accordingly, an AC voltage is induced in that coil 270. This AC voltage in turn is rectified to a DC voltage at a rectifier 682, which may comprise a standard bridge circuit. (There may additionally be data telemetry associated with the EM field 290, but this detail is ignored as impertinent to the present disclosure). The rectified DC voltage is, in turn, sent to a charge controller and protection circuit 684, which operates generally to regulate the DC voltage and to produce either a constant voltage or constant current output as necessary for recharging the battery 180.
FIG. 3 shows further details of external charger 208 with the top portion of the housing removed. Further details concerning external chargers can be found in U.S. patent application Ser. No. 11/460,955, filed Jul. 28, 2006. As shown in FIG. 3, electrical current 114 flowing in a counterclockwise direction through the primary coil 130 induces a magnetic field 290 having a prominent portion in a direction perpendicular to the plane in which the primary coil 130 lies. Primary coil 130 is typically formed of many turns of copper Litz wire, but the individual turns are not shown in FIG. 3 for clarity. Thus, when a face of the case of the external charger 208 is oriented in close proximity to an implanted device, such that the primary coil 130 is parallel to a corresponding coil within the IPG 100, the magnetic field generated by the primary coil 130 induces an electrical current within a corresponding coil to charge a battery within, or otherwise provide power, to the IPG 100.
This system is akin to a transformer where the primary coil is in the external charger 208 and secondary coil in the IPG 100. The efficiency of this coupling is largely dependent upon the alignment between the two coils, which efficiency can be expressed as a coupling factor, k. Achieving a good coupling factor is essential for optimizing efficiency of the inductive link. Not only does good coupling increase the power transferred to the implant, it minimizes heating in the implant, and also reduces the power requirements of the external charger, which reduces heating of the charger and allows a smaller form factor. Proper coupling is also essential if there is to be any data telemetry between the external charger 208 and the implant.
Operation of the external charger 208 in the prior art typically involves the use of audio feedback to the user. Thus, when charging begins, the external charger 208 produces induced field 290 and begins searching for the IPG 100, as will be explained in more detail herein. An audio transducer in the external charger 208 would provide an intermittent audible sound (e.g., beeping) when coupling was poor between the charger 208 and the IPG 100, which beeping would alert the user to move the external charger relative to the IPG. Once the positioning and coupling were improved, the charger 208 would stop beeping, and the location of the charger 208 would be held in place over the IPG 100 by using double-side adhesive pads or a belt. If the charger 208 again became poorly positioned relative to the IPG 100, the audio transducer would again start beeping, so that the position of the charger 208 relative to the IPG 100 could again be readjusted. A back-telemetry link from the IPG 100 would communicate to the charger 208 when the IPG battery was fully charged, which condition can again be audibly signaled to the patient.
As noted earlier, proper alignment between an external charger and an implant is essential for proper system function, energy transfer, and safety to the patient. However, this has heretofore been difficult to achieve. In particular, it has been noticed by the inventors that it is difficult for prior art external chargers to differentiate between a deeply-implanted device that is well aligned with respect to the charger, and a shallowly-implanted device that is poorly aligned with respect to the charger. Either scenario appears the same to the external charger 208. As a result, the patient will only know that the coupling is poor, but will not know how to remedy this situation apart from trial-and-error re-positioning of the charger.
Given these shortcomings, the art of implantable devices would benefit from techniques for achieving improved coupling between an external charger and an implantable device that provide: the ability to accurately indicate the relative position of the charger to the implant; increased charging efficiency; faster charging rates; increased patient safety and comfort; lower power requirements; and a smaller form factor. This disclosure presents such a solution.